NMR Measurement Unit Fixable Within A Process Channel

ABSTRACT

The invention relates to an NMR system, and more particularly to an NMR measurement unit including a flow channel for separating a sample from a fluid stream in a process channel, a magnet arranged relative to flow channel for creating a magnetic field in part of flow channel, a coil arranged relative to flow channel for exciting NMR active nuclei of the sample in flow channel and for receiving the frequency pulse that returns to coil from NMR active nuclei, a frame comprising a fastening flange for sealing NMR measurement unit to process channel and a chamber that is closed relative to fluid stream and connected to fastening flange, arranged to be installed mainly inside process channel, within which chamber magnet and coil are arranged and through which chamber the flow channel passes, the frame installable such that flow channel is positioned inside process channel.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims benefit of PCT/FI2020/050691 filed Oct. 21,2020, which claims benefit of Finnish Patent Application No. FI 20195912filed Oct. 24, 2019, each of which is incorporated by reference in itsentirety.

FIELD OF THE INVENTION

The invention is related to an NMR measurement unit, which includes

-   -   a flow channel comprising a first end and a second end for        separating a sample from a fluid stream present in a process        channel via the first end and for returning the sample to the        fluid stream via the second end,    -   a magnet arranged in relation to the flow channel for creating a        magnetic field at least in a part of the flow channel,    -   a coil arranged in relation to the flow channel for exciting NMR        active nuclei of the sample moving in the flow channel and for        receiving the frequency pulse that returns to the coil from the        NMR active nuclei.

The invention is also related to an NMR measurement system.

BACKGROUND OF THE INVENTION

NMR (Nuclear Magnetic Resonance) measurement can be used to determineproperties of materials for several different purposes. In the processindustry, NMR measurement can be used in the structural analysis ofmolecules and thereby, in the measurement of materials contained inprocesses.

Publication WO 2017/220859 A1 representing prior art proposes a methodand equipment for determining the beating rate of a fibre suspensionbased on an online NMR measurement. In the method, a sample is separatedfrom a process stream into a separate sampling channel passing throughan excitation coil and a magnet. Based on the Time Domain NMR

Spectroscopy, protons contained in the sample are excited and themagnitude of the frequency pulse returning from protons is measured fordetermining the characteristics of the sample.

However, a problem related to a method and equipment similar to thepublication is that the equipment requires space outside the processpipe and is difficult to move from one place to another. In addition,collecting a representative sample from the process stream can bechallenging.

U.S. Pat. No. 8,860,412 B2 discloses a methods and system for measuringnuclear magnetic resonance characteristics of formation fluid utilizingmicro-NMR sensors. The micro-NMR sensors can be used to analyze fluidflowing through the wellbore on a periodic, continuous, and/orbatch-mode basis. More efficient sampling and analysis can be conductedusing the micro-NMR sensors. In situ analysis and time-lapse logging arealso enabled.

U.S. Pat. No. 3,528,000 A discloses shows a logging tool which is usedto examine mud in a borehole. A mud sampling tube extends throughopposite side walls of the housing of the NMR logging apparatus. Apressure-tight housing surrounds the magnet, RF-coils, and the tube.

US 2016/305239 A1 discloses a downhole logging tool including an NMRmeasurement system with surface NMR microcoils located on an outersurface of the downhole logging tool. Each surface NMR microcoil has acentral axis and is distributed around the outer surface of the loggingtool with the surface NMR microcoil central axis perpendicular to thelongitudinal axis of the logging tool. The NMR measurement system mayhave a central flow line in fluid communication with the drilling fluid.Additional surface NMR microcoils or a flow line microcoil may bedisposed circumferentially around the central flow line with the surfaceNMR microcoil central axis and the flow line NMR microcoil central axis,respectively, perpendicular and parallel to the central flow linelongitudinal axis. The NMR measurement system may include a bypass flowline in fluid communication with fluid in the wellbore annulus and/orthe drill pipe.

U.S. Pat. No. 9,720,128 discloses an NMR method and apparatus foranalyzing a sample of interest applies a static magnetic field togetherwith RF pulses of oscillating magnetic field across a sample volume thatencompasses the sample of interest. The RF pulses are defined by a pulsesequence that includes a plurality of measurement segments configured tocharacterize a plurality of relaxation parameters related to relaxationof nuclear magnetization of the sample of interest. Signals induced bythe RF pulses are detected in order to derive the relaxation parameters.The measurement segments of the pulse sequence include at least onefirst-type measurement segment configured to characterize relaxation ofspin-lattice interaction between nuclei of the sample of interest in arotating frame (Tlp) at a predefined frequency. The Tlp parameter can bemeasured in conjunction with the measurement of other relaxation and/ordiffusion parameters as part of multidimensional NMR experiments.

SUMMARY OF THE INVENTION

The object of the invention is to provide an NMR measurement unit, whichcan be placed in the process in the same way as a measurement sensor sothat the sample need not be taken out from the process channel. Thepresent invention is characterized by an NMR measurement unit, whichincludes a frame comprising a fastening flange for sealing themeasurement unit to the process channel and a chamber closed at leastrelative to the fluid stream and arranged to be installed at leastmainly inside the process channel. In addition, the measurement unitincludes a flow channel comprising a first end and a second end forseparating a sample from a fluid stream present in the process channelvia the first end and for returning the sample to the fluid stream viathe second end. The flow channel passes through the chamber and theframe can be installed in such a way that the flow channel is positionedinside the process channel. The measurement unit further includes amagnet arranged in relation to the flow channel for creating a magneticfield at least in a part of the flow channel and a coil arranged inrelation to the flow channel for exciting NMR active nuclei of thesample moving in the flow channel and for receiving the frequency pulsethat returns to the coil from the NMR active nuclei. The magnet and thecoil are arranged inside the chamber.

Thanks to the frame comprising a fastening flange and a chamber, the NMRmeasurement unit can be placed inside the process channel in such a waythat the fluid stream flowing in the process channel goes through theNMR measurement unit via the flow channel and the measurement can betaken without separating the sample into separate pipelines outside theprocess channel. Due to the frame, the NMR measurement unit is easy toinstall and move in the same way as any conventional sensor designed forprocess measurement, such as a temperature sensor, for example.

Henceforth, the NMR measurement unit is referred to with a simplifieddesignation ‘measurement unit’.

Advantageously, the magnet encircles the flow channel. In this way, amagnetic field can be created with one magnet.

Alternatively, it is also possible to provide two or more magnetsarranged around the flow channel. In this case, magnets need not bespecial magnets provided with an opening at the centre for the flowchannel.

Advantageously, the coil encircles the flow channel. Thus, excitation ofnuclei and the receipt of the frequency pulse can be implemented withone coil.

Alternatively, it is also possible to provide two or more coils arrangedaround the flow channel.

Then, a large coil is not needed, for example, around a large-diameterprocess channel, but the coils can be smaller in size.

According to a first embodiment, the measurement unit includes a firstvalve and a second valve both arranged in the flow channel inside thechamber, one preceding the magnet and the other following the magnet,for stopping the flow in the region of the magnetic field. With thefirst valve and the second valve, it is possible to stop and isolate asample in the flow channel and keep it uniform and mainly in placeduring the measurement. In this way, each excited NMR active nucleusstays in the region of the magnetic field and the coil, until the pulsereflecting back from the NMR active nucleus can be received with thecoil and thus included in the measurement. In this way, it is possibleto avoid a situation in which part of protons become excited but canexit from the coil region prior to the discharge of excitation andformation of a reflecting pulse at the coil. In this context, thedefinition “preceding the magnet” means the location in the traveldirection of the sample in the flow pipe upstream of the magnet or, inother words, the location between the first end of the flow channel andthe magnet.

According to a second embodiment, the measurement unit includes a firstvalve and a pump both arranged in the flow channel inside the chamber,one preceding the magnet and the other following the magnet, forstopping the flow in the region of the magnetic field, wherein the pumpis arranged to aspirate the sample to the flow channel and stop thesample together with the first valve. With such an implementation, itcan be ensured that the sample is taken to the flow channel in the casethat the fluid stream consists of a high viscosity fluid or other thickstream, for example.

According to a third embodiment, the diameter of the flow channel iscontinuous and uniform throughout its length. In other words, themeasurement unit thus does not include valves arranged in the flowchannel, but the flow channel is of a flow-through type and themeasurement is taken from a moving fluid. Advantageously, this issuitable for fluids moving at a lower speed than 1 m/s, wherein thenuclei cannot escape through the magnetic field too fast and, on theother hand, have enough time to become sufficiently excited so that anadequately large frequency pulse can be generated at the coil.

Advantageously, in a situation according to a third embodiment, wherein,in addition, the flow speed of the fluid exceeds 1 m/s, a pre-magnetplaced in the process stream prior to the measurement unit is used inrelation to the measurement unit, enabling partial excitation of nucleialready prior to the generation of a magnetic field by the magnet of themeasurement unit, for providing sufficiently large frequency pulses atthe coil regardless of the high flow speed of the fluid.

Advantageously, the flow channel includes a pipe provided with a coatingthat prevents soiling, preferably a Teflon pipe having two ends. In thisway, the flow channel keeps clean in a better way in the region of themagnetic field, the measurement unit is more maintenance-free andsoiling does not cause an error in the measurement.

The pipe preferably includes fastening means for fastening the piperemovably to the first valve and the second valve or to the first valveand the pump. Thus, the pipe is easy to maintain and clean in case ofsoiling, without removing the entire measurement unit.

Alternatively, the measurement unit may also include a nozzle placed inrelation to the first end of the flow channel for feeding a pressurizedmedium, preferably water or air, to the flow channel at intervals forkeeping it clean.

The flow channel is advantageously composed of a pipe and a first valveand a second valve, or a pump instead of the second valve.

Advantageously, the chamber extends perpendicularly from the fasteningflange. Such a construction is easy to manufacture.

The chamber can have a round shape or preferably an elliptical shapeseen in the perpendicular direction relative to the fastening flange forreducing the flow resistance caused by the measurement unit. Thus, themeasurement unit does not notably increase pumping costs related to thefluid stream of the process channel.

The chamber may include a closable cover for closing the measurementunit. In this way, the magnetic field generated by the magnet of themeasurement unit cannot extend to the environment and, on the otherhand, components of the measurement unit are protected inside a closedchamber.

Advantageously, the fastening flange includes a bolt rim for fasteningthe measurement unit to an opening included in the process channel, suchas one or more openings of a wall of the process channel. In this way,the measurement unit is easy to fasten tightly, for example, to aninspection hatch type opening in the pipe that functions as the processchannel, instead of an inspection hatch cover.

Advantageously, the measurement unit is a time domain NMR measurementunit. Time domain measurement is a simple and relatively fastmeasurement method, thus enabling a fast measurement of the sample.

The diameter of the chamber may be in the range of 200-450 mm,preferably 300-350 mm. Thus, the flow resistance caused by themeasurement unit remains moderate.

The NMR active nucleus is preferably a proton.

The fluid stream is preferably a liquid stream. A liquid stream has asufficiently large amount of NMR active nuclei, preferably protons, sothat NMR measurement can be completed relatively quickly without a longsampling time.

The inner diameter of the flow channel may be in the range of 2-30 mm,preferably 10-20 mm. A flow channel with an inner diameter of 10-20 mmis particularly suitable for analysing liquid samples, since the flowchannel is thus sufficiently large to prevent the sample from adheringto the flow channel walls because of the surface tension between theflow channel and the sample.

Advantageously, the frame includes a lead-through for leading saidcommunication cable from the measurement unit to the computing unit. Inthis way, measurement data and control commands can be reliably takeninto a closed chamber, as the computing unit is located elsewhere.

Alternatively, if the sample is gas, the flow channel preferably has adiameter of 2-10 mm so that the NMR active nuclei over the magnet rangecan be collected closer to each other for the measurement. Whenanalysing gas, instead of a second valve, it is particularlyadvantageous to use a pump to pressurize and more intensively compactthe NMR active nuclei in the region of the magnetic field, themeasurement thus being faster.

According to an embodiment, the measurement unit can include cleaningequipment arranged in relation to the first end of the flow channel,comprising a nozzle, a pressure medium channel connected to the nozzleand a pump connected to the pressure medium channel for feeding apressure medium from the nozzle to the flow channel for cleaning it. Forexample, the pressure medium can be water or compressed air. With suchcleaning equipment, the flow channel can be cleaned from the solidmatter without removing the measurement unit from the process stream.

Another object of the invention is to provide an NMR measuring system,where the measurement unit can be installed in the process in the sameway as a measurement sensor. The present invention is characterized byan NMR measurement system, which includes an NMR measurement unit forperforming an NMR measurement on a sample separated from a processchannel for forming a measurement signal and a computing unit connectedto the measurement unit for controlling the operation of the measurementunit and computing selected properties based on the measurement signal.In the measurement system, the NMR measurement unit includes a framecomprising a fastening flange for sealing the measurement unit to theprocess channel and a chamber closed at least relative to the fluidstream and arranged to be installed at least mainly inside the processchannel. In addition, the measurement unit includes a flow channelcomprising a first end and a second end for separating a sample from afluid stream in the process channel via the first end and for returningthe sample to the fluid stream via the second end. The flow channelpasses through the chamber and the frame can be installed in such a waythat the flow channel is positioned inside the process channel. Themeasurement unit further includes a magnet arranged in relation to theflow channel for creating a magnetic field at least in a part of theflow channel and a coil arranged in relation to the flow channel forexciting NMR active nuclei of the sample moving in the flow channel andfor receiving the frequency pulse that returns to the coil from the NMRactive nuclei. The magnet and the coil are arranged inside the chamber.

In the NMR measurement system according to the invention, themeasurement unit comprises a frame, which enables the positioning of themeasurement unit at least partly inside the process pipe, thus avoidingthe need of guiding the fluid stream out from the process pipe. On theother hand, the computing unit can be located in a freely selectableposition, reachable via a connection. Henceforth, the NMR measurementsystem is referred to with a simplified designation ‘measurementsystem’.

The measurement unit advantageously includes a communication cable fortransferring control commands from the computing unit to the measurementunit and for transferring measurement data from the measurement unit tothe computing unit, and the frame includes a lead-through for leadingthrough said communication cable. In this way, measurement data andcontrol commands can be reliably taken into a closed chamber, as thecomputing unit is located elsewhere.

According to an embodiment, the measurement system is arranged todetermine the solid content of black liquor and the computing unitincludes software means arranged to compute the solid content of blackliquor using the general correlation function form SC=A*T2(R_(2TC))+B,where SC is the solid content of black liquor, T2(R_(2TC)) is thespin-spin relaxation time with corrected temperature, A is a constantand B is a constant. Most advantageously, the solid content of blackliquor is determined using the correlation function formSC=14.211n(R_(2TC))+3.05, where R_(2TC) is the temperature-correctedrelaxation time. It has been discovered that this kind of computingmethod has good accuracy.

With the measurement unit and measurement system according to theinvention, it is possible to reduce costs related to the use of NMRmeasurement in the process environment and facilitate its introductioninto use. A measurement unit using a frame is positioned inside aprocess channel, thus the sample obtained from the fluid stream beingmore representative and the measurement unit does not take space outsidethe process channel. On the other hand, a measurement unit according tothe invention is easy to install in inspection hatches or serviceopenings of an existing process channel without forming additionalopenings or channels. Furthermore, a measurement unit according to theinvention is relatively simple and maintenance-free and enables NMRmeasurement in the same way as conventional sensors. Traditionally, NMRmeasurements have been performed using separate, even offline systems,which at least guide the fluid stream out from the process channel. TheNMR measurement unit and the NMR measurement system according to theinvention are suitable for use for the analysis of both liquid andgaseous samples; however, samples are most preferably liquid, in whichcase there are more NMR active nuclei in the sample and thedetermination can be performed faster.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described below in detail by making reference to theappended drawings that illustrate some of the embodiments of theinvention, in which

FIG. 1 a is a basic view of a measurement system according to theinvention, with the measurement unit according to the invention split,

FIG. 1 b is an enlarged view, with the measurement unit according to theinvention split,

FIG. 2 is a lateral basic view of the magnet of the measurement unit,

FIG. 3 is an axonometric view of a measurement unit according to theinvention, separated and partly cut,

FIG. 4 is a bottom view of a measurement unit according to the inventionwithout the bottom,

FIG. 5 is a top view of a measurement unit according to the invention,

FIGS. 6 a and 6 b illustrate the magnet of a measurement unit accordingto the invention, separated, in different directions,

FIGS. 7 a and 7 e illustrate the frame of a measurement unit accordingto the invention in different directions,

FIG. 8 is a block diagram illustrating the operation of a measurementsystem according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

In the embodiment of the invention illustrated in FIGS. 1 a-7 b , thesample is liquid and the NMR active nuclei contained therein areprotons. In this context, it is obvious to those skilled in the art thatthe invention can also be implemented when the fluid stream is gas andthe NMR active nuclei are generally known NMR active nuclei other thanprotons, such as oxygen or phosphor.

According to FIGS. 1 a and 1 b , a measurement unit 10 according to theinvention is arranged to be used installed in a process channel 18 aspart of a measurement system 100 according to the invention in the sameway as a measurement sensor. The measurement unit 10 can be positioned,as shown in FIG. 1 a , at least partly inside the process channel 18 insuch a way that at least part of the process stream flowing in theprocess channel 18 meets the measurement unit 10 and flows through themeasurement unit 10. The measurement unit 10 is advantageously connectedto an existing process channel 18, such as a process channel related tothe processing of black liquor. In other words, a separate side flowchannel separated from the process channel is not needed for themeasurement unit.

The measurement system 100 according to the invention includes, as themain components, a measurement unit 10 and a computing unit 50, quitethe same way as in prior art measurement systems. The measurement unit10 and the computing unit 50 are preferably placed separated from eachother, thus avoiding the need to protect the computing unit 50 fromsurrounding conditions. It is advantageous to place the computing unitin the control room of a process plant, for example, or another similarroom where the conditions are favorable in regard to the durability ofelectronics, contrary to what is often the case with process channels.In this way, it is possible to increase the lifecycle of the computingunit and extend its maintenance interval, as the computing unit is notexposed to heat, vibration or dust. According to the invention, themeasurement unit 10 is integrated as part of the process channel 18,thus enabling the measurement without leading the process stream outfrom the process channel for the measurement. This results in that themeasurement system can be relatively small in size and easilyinstallable in the process channel.

The measurement unit 10 includes a frame 24 comprising a fasteningflange 26 for sealing and fastening the measurement unit 10 to a processchannel 18 and a chamber 28 that is closed at least relative to thefluid stream and connected to the fastening flange 26, arranged to beinstalled at least mainly inside the process channel 18. In addition tothe frame 24, the measurement unit 10 includes a flow channel 12comprising a first end 14 and a second end 16 for separating a samplefrom a fluid stream present in the process channel 18 via the first end14 and for returning the sample to the fluid stream via the second end16. More precisely, the flow channel passes through the chamber 28.Thanks to the fastening flange 26, the frame 24 is installable in such away that the chamber 28 and the flow channel 12 passing through it arelocated inside the process channel 18. Thus, the fluid stream flowing inthe process channel 18 meets the chamber 28 of the measurement unit 10and can be carried to the flow channel 12 via the first end 14 of theflow channel 12 and discharged from the second end 16 of the flowchannel 12 back to the process channel 18 after the measurement.

Furthermore, the measurement unit 10 includes a magnet 20, arrangedinside the chamber 28, which preferably encircles the flow channel 12for creating a magnetic field E at least in part of the flow channel 12,and a coil 22, which preferably encircles the flow channel 12 forexciting the protons of the sample moving in the flow channel 12 andreceiving the frequency pulse that returns from the protons back to thecoil 22. The magnet 20 and the coil 22 can be formed as one easilyconnectable and removable package, which is illustrated in FIGS. 4 and 6a and 6 b. In this way, the magnet 20 and the coil 22 can be easilyremoved from the measurement unit for maintenance and cleaning withoutremoving the entire measurement unit 10.

Alternatively, according to an embodiment, the measurement unit 10 caninclude cleaning equipment shown in FIG. 1 a , arranged in relation tothe first end 14 of the flow channel 12, comprising a nozzle 59, apressure medium channel 63 connected to the nozzle 59, and a pump 65connected to the pressure medium channel 63 for feeding a pressuremedium from the nozzle to the flow channel 14 for cleaning it. Forexample, the pressure medium can be water or compressed air. With suchcleaning equipment, the flow channel can be cleaned from the solidmatter without removing the measurement unit from the process stream.Instead of what is illustrated in the figure, the nozzle can also beconnected to a different point in the flow channel.

The frame 24 can be made of acid-proof steel, for example, with athickness of 4-8 mm, thus enduring the conditions prevailing in theprocess channel, such as a high pH or alternatively a low pH of liquidthat serves as the fluid stream. A metal frame 24 also prevents themagnetic field from extending to the environment and, on the other hand,disturbances external to the measurement unit from entering the magneticfield of the measurement unit. In this way, the measurement unitaccording to the invention can easily provide a closed magnetic fieldand is thus easily applicable in plant conditions. Advantageously, thefastening flange 26 has a circular shape and is dimensioned tocorrespond with the inspection hatches provided in the process channelof the application. This enables the fastening of the measurement unitdirectly to an existing inspection hatch, thus avoiding the need toprovide the process channel with new openings or lead-throughs, whichare susceptible to leaks. The fastening flange 26 includes, in relationthereto, a bolt rim 46 shown in FIG. 3 , with the bolts 58 installedtherethrough enabling the locking and sealing of the measuring unit 10to the opening 48 of the process channel 18 and to the inspection hatchof the counter bolt ring 56. A seal 61 is advantageously providedbetween the bolt rim 46 and the counter bolt ring 56 to seal theconnection.

In turn, the chamber 28 also advantageously has a circular cross-sectioncut in the direction of the plane of the fastening flange 26, as inFIGS. 5, 7 a and 7 b, or alternatively, an elliptical or oval shape, sothat the pressure loss caused by the measurement unit in the processchannel is as small as possible. Most advantageously, the shape of thecross-section is elliptical, in which case the smallest cross-section ofthe ellipse is set parallel to the flow direction for minimizingpressure losses in the fluid stream. According to FIGS. 1 a and 1 b ,the chamber 28 is dimensioned in such a way that the chamber 28 extendsinside the walls of the process channel 18 so that the flow channel 12is positioned directly in the pathway of the fluid stream. In somecases, the chamber 28 can also be dimensioned even so that the flowchannel 12 is positioned in the centre line of the process channel 18deviating from it by a maximum of 10% of the diameter of the processchannel. Thanks to the flow channel placed in the centre line of theprocess channel or near it, sampling is reliable and a representativesample can be taken from the fluid stream, as the fluid stream isuniform at the centre of the process channel.

In an advantageous embodiment, the frame 24 is so dimensioned that theheight of the frame 24 is in the range of 250-330 mm, the diameter ofthe fastening flange in the range of 400-600 mm and the chamber diameterin the range of 250-400 mm. The inner diameter of the flow channel maybe in the range of 5-30 mm, preferably 10-20 mm and the diameter of thecentre hole in the range of 30-50 mm. Since the weight of such ameasurement unit is at most 30 kg, it is easy to install and move to theapplication site.

In an advantageous embodiment, the measurement unit 10 additionallyincludes at least a first valve 32, with which the sample guided to theflow channel 12 is stopped during the measurement. In addition to thefirst valve 32, the measurement unit 10 then includes either a secondvalve 34 or, alternatively, a pump. In this case, the purpose of thefirst valve 34 or the pump is to stop the sample, together with thefirst valve 32, in the flow channel 12 by momentarily closing the flowchannel 12 at both the first end 14 and the second end 16. The purposeof stopping the sample is to ensure that protons p of the sample thatare excited thanks to the frequency pulse provided by the coil 22 alsorelease their energy in the region of the magnetic field E thus enablingthe signal that returns to the coil from the protons p to be receivedagain at the coil 22 for the measurement, as shown in FIG. 2 .

The pump is used to stop the sample particularly in the case that themeasurement unit is used to take a sample from a fluid stream that has ahigh viscosity. Thus, the pump can be used to aspirate the sample fromthe process channel to the flow channel and thereby ensure that thesample can be guided to the flow channel that has a notably smallerdiameter compared to that of the process channel. This also enables theuse of a relatively small diameter for the flow channel, since, assistedby the partial vacuum provided by the pump, the sample can be led to theflow channel in spite of the surface tension between the flow channeland the sample. If the viscosity of the fluid stream is low, a secondvalve can be used. For example, the pump can be a hose pump.

Advantageously, there is a pipe 36 functioning as a flow channel 12between the first valve 32 and the second valve 34 or between the firstvalve and the pump. The pipe 36 advantageously includes fastening means42 at both ends 40 enabling disconnection of the pipe 36 from themeasurement unit 10 for cleaning. The diameter of the flow channel canbe in the range of 2-30 mm, preferably 10-20 mm, thus allowing for theliquid sample to flow in the flow channel without problems. If thesample is gas, the diameter of the flow channel can be in the range of2-10 mm. The solid content of the sample can generally range between0.5% and 4.0% by weight remaining thus pumpable. A small diameter of theflow channel proposed above also enables the use of a smaller coil. Inthis case, the centre hole of the magnet placed on the coil,advantageously above the flow channel, can have a smaller diameter,approximately as small as between 30 mm and 40 mm. The manufacturingcosts of the magnet are generally the lower, the smaller is the holethat needs to be produced in the magnet.

Instead of the straight pipe shown in FIGS. 1 a and 1 b , the flowchannel can also be a pipe that forms a curve and has sections runningtoward the cover of the chamber. In this way, the magnet can be placedin the sections that run toward the cover of the chamber, upper from thechamber bottom, and the height of the chamber can be made lower. Themeasurement unit can also include a bypass pipe, which passes by thefirst and the second valve and the pipe between these allowing for thefluid stream that enters from the first end of the flow channel to passthe magnet during the stopping of the sample present therein. Thisreduces the flow resistance caused by the measurement unit in theprocess channel.

The size of the sample conveyed from the fluid stream to the flowchannel of the measurement unit can be as small as 1-10 cm³, in whichcase the measurement unit is also relatively small-scale. However, sucha sample is sufficient for determining a selected property of the fluidstream using the NMR measurement technique.

FIG. 2 is an enlarged basic view of an advantageous design of the magnet20 and the coil of the measurement unit, placed around the pipe 36included in the flow channel 12. The coil 22 is preferably arrangedaround the pipe 36 for exciting the protons p contained in the sample.The magnet 20 is also preferably arranged around the pipe 36 forcreating a magnetic field E in the flow channel 12. Advantageously, themagnet 20 is also arranged around the coil 22 in the radial directionrelative to the flow channel 12 above the coil 22. The magnetic field Egenerated by the magnet 20 is advantageously a magnetic field ashomogeneous and static as possible, through which the sample travelswithin the flow channel 12. The magnetic field E is depicted in thefigure with lines in the transverse direction relative to the flowchannel. The direction of the magnetic field is advantageouslytransverse relative to the longitudinal direction of the flow channel.The magnet is advantageously a permanent magnet, which can beimplemented without separate driving power in order to operate. Apermanent magnet generates a static permanent magnetic field in itself.Alternatively, the magnet can also be an electromagnet, the magneticfield of which is provided by electric current.

In addition, according to FIG. 2 , the measurement system 100 includes apower source 62 connected to the coil 22 for generating frequency pulsesand measuring equipment 70 for measuring the intensity of voltagegenerated by the frequency pulse that returns to the coil 22 fromprotons p, for forming a backward signal. Furthermore, the measurementsystem 100 includes software means 64 for determining a selectedproperty of samples based on the backward signal. Software means 64 arealso arranged to control the first valve and the second valve or thefirst valve and the pump for sampling and stopping the sample. With thepower source 62, a frequency pulse is delivered to the coil 22 to exciteprotons p travelling through the coil 22 inside the pipe 36 into ahigher energy state (spin) while the protons absorb the frequency pulse.This energy state discharges quickly (within milliseconds), the proton pthus releasing or emitting energy to its environment. Energy emitted bythe proton generates a voltage in the coil 22, i.e., a backward signal,the amplitude of which can be measured with the measuring equipment 70.

Properties of the sample can be determined based on NMR measurement bymeasuring the relaxation time between the excitation and discharge ofprotons. The relaxation time correlates with physical properties of thesample. When examining black liquor, for example, the relaxation timecorrelates with the dissolved solid content of black liquor in such away that an increased solid content changes the relaxation time so thatthe relaxation time T2 shortens as the solid content increases. Theso-called CPMG (Carr-Parcell-Meiboom-Gill) pulse sequence, whichcontains one 90° pulse and several 180° pulses, can be used to determinethe spin-spin relaxation time T2. Amplitudes of echoes of the pulsesequence attenuate according to the following equation:

a(t)=a _(o) exp(−t/T2),

where a₀ is the amplitude at the time t=0 s and T2=spin-spin relaxationtime. Parameters a_(o) and T2 can be defined by placing the equation inan experimental signal.

The measurement unit according to the invention can be implemented usingone coil or with two coils. When one coil is used, the same coil bothdelivers and receives the frequency pulse. When two coils are used, onecoil can deliver the frequency pulse and the other one receives it.However, use of one coil is also possible, when the sample is stopped inthe flow channel, thus the same protons that are exposed to thefrequency pulse also having time to deliver the backward signal in thecoil region. When implemented with one coil, the measurement unit issimpler in design compared to the use of two coils. The coil, alsocalled a bobbin, used in the device is electrically dimensioned in sucha way that, with a selected power source, it can produce the desiredfrequency pulse, or excitation pulse, in a selected magnetic field. Forexample, when the strength of the magnetic field E is 0.5 T, thefrequency pulse applied is in the frequency range of 25 MHz-26 MHz.Generally, the frequency pulse used is in the range of 50 kHz-150 MHz.When one coil is used in the measurement, the length of the coil usedmay be in the range of approximately 10-20 cm, thus protons in thesample having time to become excited and release energy over the coillength. The coil may have 100-200 turns.

Energy released by the proton p excited according to FIG. 2 provides abackward frequency in the coil 22, which can be measured as a backwardsignal. The backward signal to be measured can be measured withextremely sensitive measuring equipment 70, for example, with a receiverwhose measuring accuracy can be in the class of 1 μV. The backwardsignal to be measured is only an average signal; that is, momentaryvalues are measured for the backward signal in a certain period andbased on these values, an average is calculated for this period. Inother words, the entire spectrum is not measured, as is usually the casein spectroscopy. For example, the length of the period may be between0.5 s and 2.0 s. Based on the strength of the backward signal, therelaxation times T1 and T2 of the proton can be calculated. For example,the relaxation time can be calculated with the following formula:

T2=−t/{ln[a(t)/a _(o)}

Advantageously, software means 64 can be implemented in a computing unit50, which can be used to display results and control the device. Thecomputing unit can be a normal PC or equivalent. The material of theflow channel is preferably glass, Teflon or other equivalentnon-magnetic material, which does not disturb the generation of themagnetic field within the flow channel. In turn, the power source is anAC power source, in relation to which a frequency converter can be usedto achieve the correct frequency.

The functions of the measurement unit can be controlled with the samecomputing unit, which has software means for determining sampleproperties based on the relaxation times measured. The measurement unitcan be controlled with separate control software, which gives electricalcontrols along a field bus, for example, to the actuator 30 of thesecond valve 34, this actuator opening the second valve 34 of the flowchannel 12 for taking a sample periodically. According to FIG. 3 , thefield bus or the communication cable 52 can be led into the chamberthrough a lead-through 54 provided in the cover 44 of the chamber 28.Correspondingly, according to FIG. 1 a , electricity can be supplied tothe magnet and the coil via the lead-throughs 55 using electric wiresand a pressure medium for operating the valves via the lead-through 57.

FIG. 8 shows steps 110-126 of a measurement system according to theinvention in a block diagram. The operation of the measurement systemaccording to the invention starts from taking a sample. Advantageously,a sample is taken from the process channel 18 according to FIG. 1 a byleading a fluid stream to the flow channel 12 from its first end 14according to step 110. By opening the first valve 32 in step 112, partof the fluid stream is periodically conveyed to the flow channel 12 as asample. The sample conveyed to the flow channel 12 to the region of themagnetic field generated by the magnet 20 is stopped in step 114 withthe second valve 34 and, in step 116, the first valve 32 is closed, thesample thus remaining in the flow channel 12 between the first valve 32and the second valve 34. Periodically repeated, sampling can be repeatedat intervals of 1 to 2 minutes, for example.

The first valve 32 and the second valve 34 are advantageously controlledwith the computing unit 50 and software means 64 used in the computer inthe computing unit 50, in which the sampling interval or the volume flowper a period of time for the necessary sample stream has been specified.Based on the control software, the computing unit sends a controlcommand along a field bus, for example, preferably to the relay 68 ofFIG. 1 a , which controls the turn off power supply to the actuators ofthe first valve 32 and the second valve 34. Advantageously, the firstvalve 32 and the second valve 34 are solenoid valves, since solenoidvalves are not as sensitive to environmental disturbances as other valvetypes. When the power supply to the actuators of the valves 32 and 34 isturned off with the relay 68, the valves 32 and 34 close, and whenenergized, the valves 32 and 34 are in their open position enabling theflow of the sample in the flow channel 12.

If a pump is located in the flow channel instead of a second valve, thepower supply of the pump advantageously takes place via the same relay,the entire sampling event being thus manageable by controlling onerelay. Thus, a sample is aspirated into the flow channel until thesample is conveyed into the magnet, at which time the power supply tothe first valve and the pump is turned off with the relay, at which timethese close. At the same time, power supply to the pump is turned off.The control of the relay can be implemented with time control, forexample.

At the same time, a magnetic field has been created in the measurementunit 10 advantageously with a permanent magnet used as the magnet 20according to step 116 of FIG. 8 . The purpose of the magnetic field isto enable excitation of protons with frequency pulses generated by thecoil 22. When generated by a permanent magnet, the magnetic field ispermanent and does not require any specific control. In relation to thecomputing unit, there can also be an electronic control unit, controlledby control means, which in turn controls the power source of themeasurement unit to generate frequency pulses at the coil, according tostep 118. Frequency pulses are advantageously generated at the frequencyindicated previously while the sample is in the magnetic field.Advantageously, the frequency pulse used is the so-called CPMG frequencypulse, which includes one 90° pulse and several 180° pulses. Pulses aredelivered in succession and they excite the protons in the magneticfield in step 120. The excitation discharges very rapidly and the energyreleased by the proton arrives at the coil providing a low voltage inthe coil, which is measured with the measuring equipment in step 122.From the measuring equipment, the voltage information can be transferredin the analogue form to an A/D converter or as a digital signal directlyto the computing unit 50, where it is stored in a memory 60 with thesoftware means 64 for further processing.

The amplitude of voltage is advantageously measured continuously andmomentary measuring results of voltage are stored in the memory.Advantageously, the sample in the magnetic field is exposed to 1-20,preferably 4-8 different frequency pulses generated with the coil; thus,attenuating signals are formed in a number corresponding to that of thefrequency pulses and their amplitudes are measured with the measuringequipment. The greater the number of molecules in the sample, thesmaller can be the number of frequency pulses with which a sufficientsignal/noise value is achieved; it can be greater than 30, preferablygreater than 50. An average can be calculated of the amplitudes measuredwith the software means 64 of FIGS. 1 a and 2. In addition, an averagecan be calculated over successive samples, since variations betweenindividual samples are notably greater than variations between thesuccessive signals of the same sample.

The relaxation time T1 or T2 of the proton calculated based on themeasured amplitude of the backward signal is used together with theempirically defined correlation function to determine a selectedproperty of the sample with software means 64 in step 124.Advantageously, the correlation function is defined with empiricaltests. Based on a determination, for determining the solid content (SC)of black liquor, the following correlation function was obtained:SC=14,211n(R_(2TC))+3.05, where R_(2TC) is the temperature-correctedrelaxation time. More generally, the correlation function has the formSC=A*T2(R_(2TC))+B, where SC is the solid content of black liquor,T2(R_(2TC)) is the spin-spin relaxation time with corrected temperature,A is a constant and B is a constant. In the actual measurement, thesolid content of a black liquor sample, for example, is determined byplacing the defined relaxation time in the correlation functionaccording to step 126. The SC index can be presented as a time sequenceand an averaging process (moving averaging), for example, can beperformed to this to be able to eliminate differences between thesamples. Finally, the result of the determined solid content can betransferred to the plant information system, for example. Finally, thesample can be conveyed back to the process channel 18 from the flowchannel of the measurement unit 12 according to step 128.

In this context, the determination of the dissolved solid content ofblack liquor is presented as one example of the applications of themeasurement system and measurement unit according to the invention.However, it is to be understood that suitable applications can be, forexample, determinations of water content of black liquor and anydetermination performed on a liquid-based sample, where the samplecontains water and organic matter, thus the relaxation time correlatingwith the organic matter contained in the sample. Applications can thusbe found in studies for mining wastewaters, which contain paramagneticions, or biofuels, which do not contain water but organic compounds. Inthe case of biofuels, the relaxation time correlates, for example, withbiofuel properties, such as the cetane number or the carbon chainlength.

Equipment according to the invention, excluding the measurement unit,can consist of a commercially available prior art time domain NMRspectroscope. One such useful spectroscope is the device known by thetrade name “TD-NMR Analyzer Spin Track” manufactured by ResonanceSystems Ltd. Instead of Time Domain NMR spectroscopy, the measurementunit and the measurement system according to the invention can also beused in low-field spectroscopy.

Further modifications and alternative embodiments of various aspects ofthe invention will be apparent to those skilled in the art in view ofthis description. Accordingly, this description is to be construed asillustrative only and is for the purpose of teaching those skilled inthe art the general manner of carrying out the invention. It is to beunderstood that the forms of the invention shown and described hereinare to be taken as examples of embodiments. Elements and materials maybe substituted for those illustrated and described herein, parts andprocesses may be reversed, and certain features of the invention may beutilized independently, all as would be apparent to one skilled in theart after having the benefit of this description of the invention.Changes may be made in the elements described herein without departingfrom the spirit and scope of the invention as described in the followingclaims.

1-15. (canceled)
 16. An NMR measurement unit comprising a flow channelcomprising a first end and a second end for separating a sample from afluid stream present in a process channel via the first end and forreturning the sample to the fluid stream via the second end; a magnetarranged in connection with the flow channel for creating a magneticfield at least in part of the flow channel; a coil arranged inconnection with the flow channel for exciting an NMR active nuclei ofthe sample moving in the flow channel and for receiving a frequencypulse that returns to the coil from the NMR active nuclei; a frame beinginstallable in such a way that the flow channel is positioned inside theprocess channel, the frame comprising a fastening flange for sealing andfastening the NMR measurement unit to an inspection hatch or a serviceopening in a wall of the process channel, and the NMR measurement unitis adapted to be installed in the inspection hatch or the serviceopening, respectively, of the wall of the process channel by means ofthe fastening flange; a chamber connected to the fastening flange andarranged to be installed at least mainly inside the process channel,said chamber is closed at least relative to the fluid stream, and withinsaid chamber the magnet and the coil are arranged, and through saidchamber the flow channel passes, and wherein both the magnet and thecoil encircle the flow channel.
 17. The NMR measurement unit accordingto claim 16, further comprising a first valve and a second valve botharranged in the flow channel inside the chamber, the first valvepreceding the magnet and the second valve following the magnet, forstopping the sample in a region of the magnetic field.
 18. The NMRmeasurement unit according to claim 16, further comprising a first valveand a pump both arranged in the flow channel inside the chamber, thefirst valve preceding the magnet and the pump following the magnet orthe pump preceding the magnet and the first valve following the magnet,for stopping the sample in the region of the magnetic field.
 19. The NMRmeasurement unit according to claim 17, wherein said flow channelfurther comprising a pipe equipped with a coating that prevents soiling,having two ends.
 20. The NMR measurement unit according to claim 18,wherein said flow channel further comprising a pipe equipped with acoating that prevents soiling, having two ends.
 21. The NMR measurementunit according to claim 19, wherein the pipe comprising fasteningequipment for fastening the pipe removably by one said end to the firstvalve and by another said end to the second valve or to a pump, insteadof the second valve.
 22. The NMR measurement unit according to claim 20,wherein the pipe comprising fastening equipment for fastening the piperemovably by one said end to the first valve and by another said end tothe second valve or to a pump, instead of the second valve.
 23. The NMRmeasurement unit according to claim 16, wherein the chamber extends fromthe fastening flange perpendicularly.
 24. The NMR measurement unitaccording to claim 16, wherein, seen in a perpendicular directionrelative to the fastening flange, the chamber has a round shape forreducing flow resistance caused by the NMR measurement unit.
 25. The NMRmeasurement unit according to claim 16, wherein, seen in a perpendiculardirection relative to the fastening flange, the chamber has anelliptical shape for reducing the flow resistance caused by the NMRmeasurement unit.
 26. The NMR measurement unit according to claim 16,the chamber comprising a closable cover for closing the NMR measurementunit.
 27. The NMR measurement unit according to claim 16, the fasteningflange further comprising a bolt rim for fastening the NMR measurementunit to an opening included in the wall of the process channel.
 28. TheNMR measurement unit according to claim 27, the opening is the serviceopening or the inspection hatch.
 29. The NMR measurement unit accordingto claim 16, the NMR measurement unit being a time domain NMRmeasurement unit.
 30. The NMR measurement unit according to claim 16,wherein the NMR active nucleus is a proton.
 31. The NMR measurement unitaccording to claim 16, wherein the fluid stream is a liquid stream. 32.The NMR measurement unit according to claim 16, wherein an innerdiameter of the flow channel is 2-30 mm.
 33. The NMR measurement unitaccording to claim 16, wherein an inner diameter of the flow channel is10-20 mm.
 34. An NMR measurement system, comprising: an NMR measurementunit according to claim 16 for performing an NMR measurement on a samplein a process channel for forming a measurement signal; and a computingunit operatively connected to the NMR measurement unit for controllingoperation of the NMR measurement unit and computing selected propertiesbased on the measurement signal.
 35. The NMR measurement systemaccording to claim 34, wherein said NMR measurement unit furthercomprising a communication cable for transferring control commands fromthe computing unit to the NMR measurement unit and for transferringmeasurement data from the NMR measurement unit to the computing unit,and a frame comprising a lead-through for leading through thecommunication cable.